1. Field of The Invention
This invention relates to wireless communications, and, more particularly, to a cellular communications system with wireless units of different operating categories.
2. Description of Related Art
FIG. 1 depicts a diagram of a portion of a typical wireless communications system 10, which provides wireless communications service to a number of wireless or mobile units 12a-c, that are situated within a geographic region. The geographic region serviced by a wireless communications system is divided into spatially distinct areas called xe2x80x9ccells.xe2x80x9d Each cell is schematically represented by one hexagon in a honeycomb pattern; in practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell and other factors. A conventional cellular telephone system comprises a number of cell sites or base stations 14a-d, geographically distributed to support transmission and receipt of voice-based communication signals to and from cellular telephones, often referred to as mobile units or wireless units. Each cell site handles voice communications over a cell, and the overall coverage area for the cellular telephone system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system""s coverage area. One cell site may sometimes provide coverage for several sectors. In this specification, cells and sectors are referred to interchangeably.
A base station 14a-d comprises the radios and antennas that the base station uses to communicate with the mobile units in that cell and also comprises the transmission equipment that the base station uses to communicate with a Mobile Switching Center (MSC) 16. The Mobile Switching Center 16 is responsible for, among other things, establishing and maintaining calls between the mobile units and calls between a mobile unit and a wireline unit (e.g., wireline unit 18), which wireline unit 18 is connected to the Mobile Switching Center (MSC) 16 via a public switched telephone network (PSTN) 20. The Mobile Switching Center 16 is connected to a plurality of base stations, such as base stations 14a-d, that are dispersed throughout the geographic region serviced by the MSC 16 and to the PSTN 20 and/or a packet data network (PDN) 22, such as the Internet. The MSC 16 is connected to several databases, including a home location register (HLR) 24. The HLR 24 contains subscriber information and location information for all mobile units which reside in the geographic region of the MSC 16. Typically, for each of the mobile units, the HLR 24 stores a mobile identification number (MIN) or International Mobile Subscriber Identification Number (IMSI), the mobile directory or phone number (MDN), and/or an electronic serial number (ESN).
When active, a mobile unit receives forward-link signals from and transmits reverse-link signals to (at least) one cell site or base station. Each active mobile unit is assigned a forward link on which it receives its forward-link signals on at least one forward link channel and a reverse link on which it transmits reverse link signals on at least one reverse link channel. There are many different schemes for defining forward and reverse link channels for a cellular telephone system, including TDMA (time-division multiple access), FDMA (frequency-division multiple access), and CDMA (code-division multiple access) schemes. In CDMA communications, different channels are distinguished by different spreading sequences that are used to encode different voice-based streams, which may then be modulated at one or more different carrier frequencies for simultaneous transmission. A receiver can recover a particular voice-based stream from a received signal using the appropriate spreading sequence to decode the received signal.
In order to avoid interference between signals transmitted to and from mobile units in a CDMA system, all active mobile units within a particular cell are assigned different CDMA spreading sequences. Since cellular telephone systems are dynamic systems in which mobile units become active and inactive at different (and possibly random) times and since mobile units can move from one cell to another, the assignment of channels to the various mobile units is made by the cellular system in real time. In order to assign bandwidth resources to mobile units so as to avoid interference between mobile units in neighboring cells, the resource-assignment activities of neighboring cell sites are coordinated.
A detailed sequence of activities are typically followed before the mobile unit can access the wireless communications system to establish or receive a call. As is known in the art, calls between a CDMA mobile unit and a base station typically employ several kinds of channels. Initially, a pilot channel is employed to continually broadcast certain system synchronization and timing information to all mobile units in an area. After initial synchronization is achieved at a mobile unit, a sync channel is used to establish more specific time and frame synchronization at the mobile unit. The sync channel message also provides information about another class of channels, the paging channels. Paging channels are used to broadcast a variety of control information, including access channel information, contained in the access parameter message. This access parameter message contains parameters and other information of interest to mobile units seeking access to the base station. Other overhead messages are sent between the mobile unit and the base station to facilitate communications over forward and reverse link traffic channels between the mobile unit and the base station over which voice and/or data information is transmitted.
Conventional CDMA systems being developed use macrodiversity to improve the performance or quality of reception. The macrodiversity concept as is used in CDMA involves two or more simultaneous links from two or more base stations. The mobile and cell receivers employ a number of parallel correlators. Receivers using parallel correlators (sometimes called RAKE receivers) allow individual path arrivals to be tracked independently and the sum of their received signal strengths is then, used to demodulate the signal. While there is fading on each arrival, the fades are independent. Demodulation based on the sum of the signals is then much more reliable. However, this process may use a lot of the resources/equipment of a cell site. Moreover, it is expected that quite a high percentage of the calls will use macrodiversity.
Once a system is designed, the number of transceiver elements are fixed, and as the number of users increase, macrodiversity will consume the wireless resources. For the macrodiversity to operate, the mobile unit searches for pilot signals of base stations on a candidate list which are not currently servicing the wireless unit. The wireless unit measures the signal strengths, for example using a received signal strength indicator (RSSI) of the pilot signals. When the wireless unit detects a pilot of sufficient strength which is associated with a base station not assigned to the wireless unit, a pilot measurement message including the pilot signal measurements is provided to the MSC 16. Using the pilot signal measurements, the MSC 16 determines whether to assign a traffic channel from another base station to the wireless unit. The wireless unit is said to be in macrodiversity state if it is assigned traffic channels from more than one base station. Within the geographic region, the MSC 16 switches calls from one base station to another in real time as the mobile unit moves between cells, referred to as call handoff.
All base stations connected to a given wireless unit define the active set of that mobile unit, and an active set update function controls, i.e, evaluates and updates, this active set based on pilot strength measurements. At the wireless unit, the strongest pilots are detected and measured by the measurement process. The signal strength values are then collected into the pilot measurement report, which is sent to the MSC 16. When the active set update function is invoked, the base stations within the active set from which the strongest and weakest pilot are received are identified. If the difference is greater than the system desired value (called active set window thereafter), the weakest base station will be removed from the active set. A base station is added to the active set window if the received pilot signal strength is within the window and above an active set threshold value, provided the active set size (no. of RAKE fingers in the receiver) is not exceeded. If the active set size is full, the weakest base station in the active set will be replaced by the new base station if the corresponding pilot signal strength is higher than the weakest base station.
Some wireless cellular communications systems involve fixed wireless units. The fixed cellular concept is getting a lot of attention and is a substitute for the public switched telephone network (PSTN), where the terrain is difficult and the infrastructure cost is too high to implement the PSTN. The fixed cellular concept involves wireless units where the mobility of the wireless unit is very limited within the home cell (limited or no mobility at all). Macrodiversity is not normally used and no handoffs are allowed between the base stations.
Macrodiversity as used in the CDMA system has the advantage that it improves the quality of reception in the cellular or PCS system, but if the cellsite is fully loaded (all the transceivers are being used), the capacity of the system suffers. Thus, macrodiversity needs to be coordinated, especially in a system using both fixed and mobile units, to more efficiently use the resources of the wireless communications system.
The present invention involves a macrodiversity control system, for a wireless communication system, which provides a wireless unit using a macrodiversity mode depending on the operating category of the wireless unit. For example, a wireless communications system can have wireless units operating in categories related to the mobility of the wireless unit, such as fixed, limited fixed (mobility within the home cell) and/or fully mobile. For a wireless unit, the macrodiversity mode of the wireless unit can be established depending on the operating category for the wireless unit. Where the wireless unit is fixed and has good reception from a home base station, particularly in regions close to the home base station, macrodiversity for the wireless unit can be restricted. Because the mobility of the wireless unit is restricted in the fixed (or limited fixed) mode, the fading of signals between the home base station and the wireless unit should not be severe, thereby macrodiversity is disabled. Where the reception is poor, particularly in a region near the edge of the cell, the macrodiversity for the fixed (or limited fixed) wireless unit is enabled. The fixed (or limited fixed) wireless unit at the edge of the cell can benefit from macrodiversity, but because the wireless unit is fixed (or limited fixed), the fixed (or limited fixed) wireless unit is not handed off to another base station (or can only be handed off to a limited set of base stations).